1. Field of the Invention
The present invention relates to a method of manufacturing a vertical-cavity surface-emitting laser (VCSEL) device and the VCSEL device.
2. Description of the Related Art
As a conventional vertical-cavity surface-emitting laser (VCSEL) device, Japanese Patent Application Laid-Open No. 2004-319643 discloses a VCSEL device obtained by growing an active layer between an upper semiconductor multilayer mirror and a lower semiconductor multilayer mirror, to form a distributed Bragg reflector (DBR) laser, and further forming a current confinement layer for confining a current path to increase the current injection efficiency. The current confinement layer is formed by selectively oxidizing an AlAs layer, and includes a circular current confinement region of AlAs located at the center and a selectively-oxidized region of aluminum oxide located around the current confinement region. The current confinement region works as a current path when a current is injected into the VCSEL and an aperture area through which a laser light is emitted.
When the current confinement layer is formed by selectively oxidizing the AlAs layer, there is a case where anisotropy occurs in the oxidation rate, causing a shape of the current confinement region to be a rhombus, and the current density is concentrated to an edge of the rhombus to make an origin of a dislocation. To solve this problem, a technology is disclosed in K. D. Choquette et al., “Advances in selective wet oxidation of AlGaAs alloys”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 3, No. 3, pp. 916-926 (1997), in which the anisotropy of the oxidation rate is resolved by using an AlGaAs layer as a selective oxidation layer instead of the AlAs layer.
However, the AlGaAs layer has a small composition ratio of Ga, and thus it is difficult to control the composition ratio to a desired value. A technology to solve the problem is disclosed in G. W. Pickrell et al., “Improvement of wet-oxidized AlxGa1-xAs (x to 1) through the use of AlAs/GaAs digital alloys”, Appl. Phys. Lett., vol. 76, No. 18, pp. 2544-2546 (2000), in which the anisotropy of the oxidation rate is suppressed in a well-controlled manner by employing AlAs/GaAs digital alloys as a selective oxidation layer, which is formed by alternately growing an AlAs layer and a GaAs layer.
If the thickness of the AlAs layer is thick, a stress occurring in a selectively-oxidized region when the AlAs layer is selectively oxidized to form the current confinement layer becomes large. Consequently, the reliability of the VCSEL may decrease; and therefore, the thickness of the AlAs layer is required to be as thin as possible. It is reported in K. D. Choquette et al., “Advances in selective wet oxidation of AlGaAs alloys”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 3, No. 3, pp. 916-926 (1997) that there is a relationship between the thickness of the AlAs layer and the oxidation rate of the AlAs layer when the current confinement layer is formed by selectively oxidizing the AlAs layer. The oxidation rate mentioned here indicates an oxidation rate in a direction parallel to the main surface of the AlAs layer.
FIG. 11 is a graph of the relationship between the thickness and the oxidation rate of the AlAs layer. If the thickness of the AlAs layer is thin, as shown in FIG. 11, the oxidation rate rapidly changes with the change in the thickness of the layer. Namely, the oxidation rate is highly layer-thickness dependent. Consequently, if a large number of VCSELs are formed on a single substrate, even though a difference in thicknesses of the AlAs layers in a substrate plane is a slight amount, a difference in the oxidation rate between the lasers becomes significant. This causes fluctuation in diameters of current confinement regions of the VCSELs.